• When magma reaches the surface it loses its gases and
become lava
• The igneous rocks that form on the surface are called
extrusive or volcanic (after the fire god Vulcan).
• Magmas that fail to reach the surface crystallize below
the surface and produce the intrusive or plutonic rocks
(after Pluto, the god of the lower world).
• Plutonic rock would only appear on the surface of the
Earth when crust is uplifted and the overlying rocks are
eroded. The exposures of these rocks are called outcrops
Igneous Rocks
Igneous Rocks
Formed by cooling and
hardening of magma
•Plutonic –
intrusive
,
forms underground
•Volcanic –
extrusive
,
forms on surface
Two kinds of magma
• High SiO2%, light
colored, thick, slow
moving
•Low SiO2%, dark
colored called mafic
Igneous Rocks
•Grain size and
texture depends on
how quick cooled
–Slow = large
crystals
–Fast = small crystals
Igneous Rocks
•Grouped based on
mineral composition
–Light = granite
–Dark = gabbro
Igneous Compositions
Granite: quartz + alkali feldspar
Coarse grained = phaneritic
Granites intruded by dikes
NE Aqaba
Rhyolite (quartz + alkali
feldspar)
fine-grained = aphanitic
Andesite porphyry
two distinct sizes
Scoria
Pumice
Obsidian
Diorite
Porphyritic granite